Aflatoxin-B1 (AFB1) is the most potent member of the aflatoxin family of hepatocarcinogens. The DNA adducts of AFB1 inhibit replication, transcription and translation, and thus are highly lethal to cells. In its activated form, AFB1 binds to the N7 of guanosine as a population of adducts, the most plentiful of which is 8,9-dihydro-8-(N7-guanyl)-9- hydroxyl-aflatoxin-B1 (AFB1-N7-G). In this adduct, the AFB1 creates a positive charge at N7 that not only destabilizes the guanine base, but also causes a severe 36 degree bend in the DNA helix. To alleviate the positive charge, the imidazole ring can open to produce a chemically stable formamidopyrimidine adduct (AFB1-FAPY) that stabilizes the DNA helix against denaturation with a minimal structural effect. This project is an analysis of the ability of DNA repair systems to act upon the aflatoxin adducts both in vitro and in vivo. Current data suggests that bacterial systems can remove both AFB1-N7-G and AFB1-FAPY adducts. In mammalian systems, by contrast, the AFB1-N7-G adduct is removed, but the AFB1-FAPY lesion is more persistent. This persistence, along with their high chemical and thermal stability, suggest AFB1-FAPY lesions may be a major progenitor of AFB1 toxicity and carcinogenicity. The first part of this study will evaluate the relative abilities of bacterial and mammalian repair enzymes to remove the two types of AFB1 adducts. In additional work, host cell reactivation assays will probe the relative gene inactivation abilities of the two adducts and, in addition, determine how the expression of specific repair proteins (e.g., the mammalian homologue of a bacterial AFB1-FAPY glycosylase) affects lesion tolerance in prokaryotes and eukaryotes. An understanding of why some lesions persist and others are removed in various cells may contribute toward the developing picture of how aflatoxin adducts promote cell death, mutation, and, ultimately, carcinogenesis.